Choose a ripe, sweet watermelon and cut into cubes, discarding the rind. For optimal sweetness, refrigerate watermelon before cutting.
Place watermelon cubes in a blender or food processor. Add honey, lime juice, lime zest, and salt. If using vodka, add it now. Blend until completely smooth, about 30-45 seconds.
For ultra-smooth sorbet, strain the mixture through a fine-mesh sieve to remove any pulp or small seed fragments (optional).
Pour the mixture into a shallow metal pan. Cover with plastic wrap, pressing it directly onto the surface to prevent ice crystals from forming. Freeze for 2 hours until edges begin to solidify.
Remove from freezer when partially frozen (consistency should be similar to a slushy). Using a fork or spatula, vigorously break up any ice crystals that have formed. Return to freezer for 1 hour.
After another hour, transfer the partially frozen mixture back to the blender and pulse until smooth. Return to the freezing container and freeze until firm, about 2-3 more hours.